Science Musings Blog

Friday, February 11, 2005

Earthshine

Two nights ago, Jennifer in California saw the day-old young Moon that Tom and I missed because of clouds -- a few hours older in California than on the east coast, but still wonderfully thin. Last evening the Moon here in the Bahamas was two days old, a silver sliver in a clear tropical sky.

A two day old Moon is ideal for seeing what is called "the old Moon in the young Moon's arms" -- a brilliant crescent, and the rest of the Moon's disk bathed in pale light. The illumination on the "old Moon" is earthshine, sunlight reflected from the Earth -- which appears almost full from the Moon -- then back again. Because of the extra Earth-Moon round trip, the light from the "old Moon" is a few seconds older than the light from the crescent.

Tonight the earthshine will be less obvious because the fatter, brighter crescent will tend to obscure it, and because the Earth will be less full as seen from the Moon.

Also, earthshine on the Moon can vary from month to month depending on how much cloud and ice cover there is on the side of the Earth facing the Moon; clouds and ice reflect more light than sea or land.